Radio receiving system



y 1941- R. s. OHL 2,242,791

RADIO RECEIVING SYSTEM I r Filed Sept. 1, .1939 2 Sheets-Sheet 1 FIG. 2

FIG. 4

INI/ENTOR R. S. OHL

ATTORNEY May 20, 1941.

RADIO RECEIVING SYSTEM Filed Sept. 1, 1939 R. s. OHL

2 Sheets-Sheet 2 FIG 6' I. EA.v

ATTORNEY ld-UfP-ill Patented May 20, 1941 RADIO RECEIVING SYSTEM Russell S. Ohl, Little Silver, N. .L, assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application September 1, 1939, Serial No. 293,010

12 Claims.

This invention relates to reception of modulated carrier waves and more particularly to methods and apparatus for carrier wave reception employing devices having cube law current characteristics.

An object of the invention is to transfer the modulations from an incoming carrier Wave to a fixed frequency locally produced carrier Wave without first demodulating the incoming carrier wave.

Another object of the invention is to enable incoming modulated oscillations to give rise to similarly modulated fixed frequency oscillations of a lower frequency which is not a function of the incoming carrier wave frequency and which may therefore be selected by fixed intermediate frequency circuits requiring no variable tuning appliances. I

A further object of the invention is to provide a carrier wave system of the intermediate frequency type in which both the local carrier oscillator and the intermediate frequency selective circuit require no variable tuning.

Another object of the invention is to provide a simple manually operable volume control for a radio receiver equipped with automatic gain control.

In accordance with the invention a carrier wave receiver is provided with a local source of intermediate frequency oscillations connected to a balanced or bridge circuit such that the normal alternating electromotive force across the output terminals of the bridge which are conjugate to the terminals connected to the oscillator is zero. One arm of the bridge comprises an impedance element as, for example, a contact rectifier including a body of silicon which has a substantially cubic relation between output current and impressed electromotive force. The incoming modulated oscillations are applied to the bridge arm having the cubic characteristic and serve to unbalance the bridge and to permit oscillations from the local source to pass to the output circuit. One component of the resulting output current comprises oscillations of the local carrier frequency modulated in amplitude by the square of the signal modulations carried by the incoming modulated oscillations. This component is subjected to a rooting action by which the amplitude of the modulation is derived from the squared signal modulation to provide a signal modulated intermediate frequency carrier wave which may thereafter be demodulated in the usual manner.

In one embodiment of the invention, a radio receiver employing the bridge modulator unit for transferring modulations from an incoming carrier wave to a locally produced or intermediate frequency carrier wave is provided with an automatic gain control system associated with the intermediate frequency amplifiers. In order to manually change the volume level as desired the bridge modulator unit is provided with a manually controlled potentiometer which permits varying the bridge in such manner as to unbalance it so that the ratio of unmodulated to modulated intermediate frequency carrier is varied, thus enabling the desired volume level to be varied at will without any manual readjustment of the automatic volume control apparatus.

In the drawings:

Fig. 1 discloses a radio receiving system in which the invention is employed to transfer signal modulations of an incoming wave to an intermediate frequency carrier wave without an intervening demodulation;

Fig. 2 is a graph of the current output characteristic of the modulation transfer device of the system. of Fig. 1;

, Fig. 3 illustrates a modification of a portion of the system of Fig. 1;

Figs. 4 and 5 are modifications of the radio receiving system of Fig. 1; and

Fig. 6 is a schematic circuit diagram of a radio receiver equipped for automatic gain control and for manual control of the volume level.

Fig. 1 illustrates an embodiment of the invention in a radio receiving system. An antenna or high frequency circuit I is connected to ground through the variable tuned circuit 2 which is made resonant to the frequency of the incoming oscillations which it is desired to receive. Connected to the circuit 2 by transformer 3 is a carrier transfer modulator device 4 of the contact rectifier type 'which may comprise a body of vacuum fused highly purified silicon with which is associated a point contact element. The device 4 constitutes one arm of a four-arm bridge including the two portions 5 and 6 of the secondary winding of the transformer l and an impedance arm consisting of inductance 8 and variable resistance 9. A source IU of intermediate frequency oscillations is connected to the primary winding of transformer I. The bridge 4, 5, 6, 8, 9 is normally (i. e. during absence of incoming carrier waves) so balanced at the intermediate frequency that no difference of potential exists between the point II and ground l2 for the intermediate frequency since during absence of incoming carrier waves these points are conjugate to points l3 and 14 across which the intermediate frequency electromotive force is impressed.

Connected from point H by way of a large stopping capacity element I5 and a band-pass filter It is a path ll which divides at 18, one branch passing by way of the primary winding of transformer I9 to ground, the other including a crystal detector or contact rectifier which may preferably consist of a body of iron-pyrites with an appropriate point contact element. The rectifier 2%] is shunted by a fixed resistance element 2 I. A large capacity stopping condenser 22 completes the alternating current path to ground while the source 23 of unidirectional polarizing electromotive force is connectedin series with variable resistor 24 to ground at 2-5. Coupled to the secondary winding of transformer l9 is the input or grid-cathode circuit of a thermionic detector 28 in the output current path of whichis a loud-speaker or other speech indicating device or transmission circuit 29.

Assume, for example, that oscillations of a carrier frequency 01 which have been amplitude modulated by signals of frequency 8 are to be received and that it is desired to transfer the modulations of the received wave to a locally produced wave of frequency 02. The antenna l and the selective circuit 2 will, in accordance with the usual practice, he designed to select oscillations of a frequency C1 or a desired band 0 of frequencies in that region. The selected incoming modulated carrier wave may be represented by Ei Ai sin arcs (1+3 sin also where 21825 is the angular frequency of the instantaneous modulating tone, B its amplitude and A0 is a function of the amplitude of the unmodulated carrier wave at the transmitting station. The modulated incoming wave is impressed through transformer 3 upon transfer modulator 4 over a path including the capacity element 30 which presents small impedance at the incoming carrier wave frequency 01 but which is of relatively high impedance at the frequency 2 of the locally generated wave. Simultaneously there is impressed upon the transfer modulator 4 an electromotive force produced by the local source l9 which may be represented by E321) sin ar-C21,

The transfer modulator 4 has a non-linear characteristic relating output current to total im pressed electromotive force as illustrated in Fig. 2 and indicated by the equation associated with that figure.

It is only n cessary that A3 be of the same order of magnitude or preferably greater than the other coefficients. It is apparent from Equation 3 that the output current of the device 4 will involve a number of components. Since by suitable selective circuits, such as the band-pass filter Hi, we may select any of these components excluding the others we may legitimately drop those not of interest and confine our interest to a particular one. If, therefore, we consider only the purely cube term of the second member of Equation 3 The total electromotive force E impressed upon the transfer modulator 4 is the sum of that of the received modulated'wave and that of the local source and may be expressed by Substituting the value of E from Equation 5 in Equation l Again, applying the principle that the use of proper selective circuits permits dropping of terms expressing components of frequencies excluded by the selective circuits and retaining onlythe second term of the right-hand member of Equation 6 the selected output current is expressed by I=3A3E12E2+ (7) Substituting in Equation '7 the magnitudes of E1 and E2 as given in Equations 1 and 2,

sin 21rc t(1+B sin amo 2 cos 41rc t sin 27rC i(1+B sin 21rst) (9) The second term of the right-hand member of Equation 9 involves the double carrier frequency 41Tclt which is filtered out, leaving I=% sin 21rC i(1+B sin 21rst) (10) The resulting selected output current accordingly has an amplitude factor a carrier wave frequency 02 and an envelope or modulation factor 1+B sin 21rst) which is the square of that of the received carrier wave electromotive force as indicated by Equation 1. Since the band-pass filter J6 is made selective for a band of frequencies within the region of the local carrier frequency 02 there will appear in the path I1, as expressed in Equation 10 a modulated wave of the carrier frequency 02 having a modulation corresponding to the square of the modulation of the incoming carrier wave.

The device 20, together with its associated elements 2|, 22, 23 and 24 performs the function of a rooterf which produces a response which instead of being proportional to the squaredsignal amplitude of theimpressed intermediate frequency carrier waves is generally proportional to its square root. The device 26 is a contact rectifier of iron-pyrites or the like and preferably has a square law characteristic. Both its charintermediate carrier frequency and the modulation of which varies in amplitude in accordance with the modulations of the original incoming carrier wave.

It is not essential that the rooter be of the particular type disclosed since devices having an analogous function as disclosed in U. S. patents to Crisson, 1,737,830, December 3, 1929, and Doba 2,118,175, May 24, 1938, may be employed.

After the rooting operation the resultant electromotive force may be impressed upon the input circuit of the demodulator 28 from the output circuit of which signal currents are transmitted to the loud-speaker or transmission line 29.

Fig. 3 illustrates a modification of that portion of the circuit of Fig. 3 beneath the linezva: of Fig. 2. A full wave rectifier consisting of two reversely poled iron-pyrites units and 26 takes the place of the elements 20, 2| of Fig. 1. v

Fig. 4 discloses a modified receiving system for modulated incoming carrier waves in which a local carrier source is connected to the input circuits of two triodes having cubic characteristics, the output circuits of the triodes being connected in difierential or opposing relation to a common output amplifier so that normally in the absence of incoming carrier waves no oscillations from the local source pass to the common output amplifier. This balanced condition is upset, however, whenever incoming carrier waves are received since the incoming carrier waves affect the potential of the grid of one triode but not that of the other.

' Referring to the drawings the source of local or lator. Oscillations from source 32 are transmitted by way of transformer 33 and the leads 34 and 35 to the input circuits of the cubic characteristic triodes 36 and 31. The magnitude of the electromotive force so applied is controllable by a potentiometer 38. The electromotive force is applied effectively in parallel to the grids of the two triodes over a path through the low impedance potentiometer 38 to point 39. The path leads from point 39 to the grid of the triode 36 by way of a grid leak and blocking condenser combination 40. Another branch of the path for the locally produced oscillations leads from the point 39 by way of loop antenna M and the grid leak and condenser combination 42 to the grid of triode 31. The output circuits of the cubic characteristic triodes 36 and 31 are connected in differential or opposing fashion by a transformer 43 to the intermediate frequency amplifying system comprising the tandem amplifiers 44 and 45. .The

common plate current circuit of triodes 36 and 31 extends by way of a variable tap connection, which permits a nice balance to be obtained. In order to prevent feedback from the plate circuit to the input circuit of the discharge devices, the plate of each device is cross-connected through a neutralizing or balancing condenser to the grid of the other device. I Y

Incoming modulated carrier waves are received by the loop antenna 4! which together with the variable capacity element 61 is tuned to the desired carrier frequency. The electromotive force of the incoming carrier waves developed across capacity element 41 is impressed upon the input circuit of device 3! the terminals of which are connected to those of. the capacity element Sub stantially no incoming carrier frequency electromotive force is impressed upon the device 36 since the grid of device 36 is connected to the point 39 which is in turn connected to earth by way of the low impedance potentiometer 38, the lead 34 and the ground connection 48. Accordingly when incoming modulated carrier waves are received and affect only the device 31, the balance of devices 36 and 31 is disturbed and oscillations of the locally generated or intermediate carrier frequency produced by source 32 are impressed upon the tandem amplifier M, 45. These oscillations have amplitude modulations corresponding to the square of the modulations of the incoming carrier frequency waves in accordance with the principle of operation explained in connection with the previously-described system of Fig. 1. The circuits of amplifiers 46 and 45 may be made selective to the intermediate frequency carrier wave by wellknown expedients and the tandem amplifiers will therefore select and pass on to the detector 49 modulated intermediate frequency carrier waves which upon demodulation by the detector yield speech currents to the loud-speaker or other speech translating device 55. The detector 49 may be given a rooting characteristic by proper design of its grid leak path.

Fig. 5 illustrates another embodiment of the invention in a radio receiving circuit in which an antenna or high frequency receiving conductor 54 is associated with selective tuned circuit 55 and is connected to ground through the stopping condenser 56 and resistor 51. Connected across the terminals of the variable capacity element 58 of the tuned circuit 55 are the input terminals of a radio frequency amplifier 59 perferably of the screen grid type, the tuned output circuit 60 of which is connected by transformer 6| to a contact rectifying device 62 similar in its construction and characteristic to the cube law characteristic device 4 of Fig. 1. The device 62, together with the secondary winding of the transformer 6| and condenser 35, constitutes one arm of a bridge circuit another arm of which comprises the inductance 63 and variable resistor 66. The remaining two arms consist of the two portions 65 and 66 of the transformer 6'! by which intermediate frequency carrier oscillator 68 is coupled to the bridge. The output terminals of the bridge are connected through a stopping condenser 69 to a contact rectifier rooter l6 which may correspond in structure, characteristics and function to the square law device 21'! of Fig. 1. Connected across the terminals of device 10 through the large capacity element H is a transformer E2, the secondary winding of which leads to the input terminals of intermediate frequency amplifier 13, the output circuit of which is coupled to the rectifying system of diode ll.

4 Intermediate frequency amplifier '13 is provided with a by-pass condenser E5 to short-cirtransferred through a series capacity element 86 to a second rectifier Bi. The resulting unidirectional potential across rectifier 6! is additionally smoothed by series inductance 32 and resistance 83 which cooperate with the shunt capacitances of radio frequency amplifier 59.

In operation, incoming radio frequency oscillations received by circuit 54 and selected by tuned circuit 55 are amplified by amplifier 59 and again selected by tuned circuit 69 by which they are impressed upon the carrier transfer modulator 62, together with intermediate frequency oscillations from the local source 69. As has been explained, the device 62 is included in a bridge so balanced that normally no energy from the source 58 reaches the output circuit connected to conjugate points 85 and 86 of the bridge. However, application of the input carrier oscillae tion electromotive force to the device 52 with its cubic order characteristic so alters the impedance of device 52 that the bridge is no longer balanced and'the unbalance manifests itself as an electromotive force across the points 85 and 86. This unbalance electromotive force is accordingly impressed across the rooting device 70. As has already been explained, the unbalanced output energy of the bridge contains a component of the intermediate carrier wave frequency modulated in accordance with the square of the modulations of the initially received modulated high frequency oscillations. Modulated incoming frequency car,- rier waves are operated upon by the device 79 to derive the signal modulated intermediate frequency carrier waves, the resulting signal modulated currents are impressed upon the intermediate frequency amplifier l3 and after amplification are demodulated by diode rectifier H. The demodulated signal current electromotive force impressed across the resistance 18 gives rise to currents which are transferred to the outgoing circuit connected to the secondary winding of transformer 14 for retransmission as may be desired. Speech current energy applied by the rectifier Ti to the rectifier 8| is utilized after smoothing of its potential by the smoothing filter to provide an automatic volume control potential for the grid of the radio frequency amplifier 59 and, also, if desired for the grid of the intermediate frequency amplifier '53. This feature is of particular advantage in connection with the carrier transfer modulator and the rooter since control of the volume of energy supplied to these devices insures; that they operate in the most eificient region of their load characteristics without overloading. Control of the level of the intermediate frequency amplifier output performs a corresponding function for the diode rectifying system 11.

The radio receiver circuit of 6 includes an antenna 88, a band-pass filter 99, a bridge with a local intermediate frequency carrier source 99 and a cube law carrier transfer modulator 9|, a three-stage intermediate frequency amplifier 92, a full wave rectifier type of demodulator 93, a balanced or push-pull signal or speech current amplifier 94 and a signal indicating device 95. The unidirectional current from device 93 traverses' the resistance between points 95 and 9'! with the polarity indicated. The negative potential at 91 is impressed by way of resistance 98 and the individual choke coils 99 to the grids of the first two amplifiers of the intermediate frequency amplifier system. Accordingly, the amplification introduced by the amplifier system 92 and the resulting signal volume in the indicating device 95 are held at a regulated level in well-known manner. This level, as is well known, is dependent upon the relative amplitudes of the unmodu- 84'and56 as a smoothing filterforthe rectified, electromotive force to beimpressed onv the-grid latedmomponent and; the, modulated component of: the-intermediate. frequency carrier wave as impressed: uponthe intermediate frequency amplifier system 92.

The-:level at whichthe automatic volume control device holdsthefinal signal output as indicated; by receiver 95 may be changed as desired by'varying the relationship between the unmodu lated andvmodulated components of the intermediate ,-fre.quency carrier wave. The bridge including the;carrier transfer modulator 9! is provided with a potentiometer input for the local carrier wave so that the bridge may be unbalanced in varying degree at will. In operation the potentiometer of the bridge may be set so that part I presents aresistance of about 100 ohms while part Hll has-avalue of say 900 ohms. The bridge arm: including capacitance I02 and the volume control resistor I03 is adjusted for best balance when no signal is being received on the antenna. The grid bias electromotive force Eg is at its minimum magnitude for this condition. Upon receipt of incoming carrier waves the balance of themodulation transfer bridge is upset and Eg increases so as to hold the receiver output at a predetermined level. If the resultant signal is deemed. too intense the operator may slightly adjust the magnitude of the volume control resistor I03 to unbalance the bridge to the desired degree. This changes the relation between the unmodulatedand the modulated components or the percentage modulation as it is usually termed. The automatic volume control accordingly changes the biasing potential on the grids of the amplifier system 92 to bring the output level at receiver 95 to the correct magnitude.

Although a rooting device may be employed in this circuit, as in those previously described, it is not an essential. The rooting operation may be accomplished at the transmitter in the general'manner of volume compression and expansion systems commonly known as compandor systems. If the distortion of the speech or other signal is not toogreat it is feasible to dispense with the rooting operation altogether.

It will be understood that although only single intermediate frequency stage conversion systems have been disclosed the principle of the invention may equally well be applied to multistage frequency transforming receivers in which several intermediate frequency stages, each employing its own intermediate carrier frequency source, are utilized. The invention may also be employed in an apparatus for stepping up the carrier frequency at transmitting stations or for changing the carrier frequency in any desired manner at a repeating station. In general, the invention may be employed to transfer signal modulations from one carrier wave to another carrier wave without an intermediate step of demodulation to the signal frequency. Moreover, the frequency of the new carrier wave to which the signal modulations are transferred may be entirely independent of any frequency characteristics of the carrier wave from which the signal modulations are transferred.

What is claimed is:

1. The method of modulated carrier wave reception which comprises subjecting an incoming modulated carrier wave and a locally produced wave to an interaction having a cube law characteristic and selecting from the resultant oscil-" lations of the frequency of the locally produced wave having modulations corresponding approximately to those of the incoming wave.

-2. In combination, a device having a response characterized by a cube law term, means for impressing thereon received incoming oscillations, a source of locally produced oscillations of a fixed frequency connected thereto whereby incoming oscillations and locally produced oscillations are caused to interact therein, and an output circuit connected to said device selective for oscillations of the locally produced frequency.

3. In combination, a source of oscillations of a fixed intermediate frequency, a bridge circuit connected thereto and so balanced that between two fixed points in the circuit conjugate to those of the oscillator connection the normal potential difference at the intermediate frequency is zero, the bridge comprising a plurality of arms one of which has a non-linear impedance characteristic, means for impressing received modulated high frequency oscillations upon the non-linear impedance bridge arm, and means connected to the fixed conjugate points selective to a band of frequencies including that of the intermediate frequency oscillator to derive therefrom modulated intermediate frequency oscillations having the carrier frequency of the intermediate fre- -i an automatic volume control device for controlling the gain of the amplifiers, and means inde pendent of the amplifiers for varying at will the relative magnitudes of the modulated and unmodulated carrier components impressed upon the amplifiers to enable the volume level to bev s.

varied as desired.

5. An intermediate frequency type carrier wave receiver comprising a Wheatstone bridge balanced at a preassigned frequency, a cube law device constituting an element of one arm of the bridge, an input circuit for receiving incoming waves and arranged to supply energy of said waves to the cube law device, a local source of oscillations of the preassigned frequency connected to two diagonally opposite terminals of the bridge to intermodulate with incoming waves in the cube law device, an amplifier having an automatic gain control connected to the other two terminals of the bridge, and means for varying the normal balance of the bridge to vary the relation between the intermodulated oscillations and the unmodulated oscillations of the preassigned frequency which are passed by the bridge to the amplifier.

6. The combination of claim 2 characterized in this, that the device consists of a contact rectifier comprising a small point conductor bearing against a body of vacuum fused silicon of a high degree of purity.

'7. In combination, a local source of oscillations of a fixed intermediate frequency, a bridge circuit coupled thereto and normally so balanced that between two fixed points in the circuit, conjugate to the points of coupling to said source, the normal potential difference at the local source frequency is zero, the bridge having a plurality of arms at least one of which contains an element responsive to received modulated waves and adapted to upset the balance of the bridge in response thereto, means for impressing received modulated waves on said element, a circuit arrangement selective to a band of frequencies including the frequency of the local source connected to the fixed conjugate points to derive therefrom modulated intermediate frequency cscillations having the carrier frequency of the local source and modulations corresponding to those of the received wave, at least one arm of the bridge containing a manually variable impedance element for altering at will the amount by which received waves of a particular strength will upset the balance of the bridge.

8. In combination, a local source of oscillations of a fixed intermediate frequency, a bridge circuit coupled thereto and normally so balanced that between two fixed points in the circuit, conjugate to the points of coupling to said source, the normal potential difference at the local source frequency is zero, the bridge having a plurality I of arms'at least one of which contains an element responsive to received modulated waves and adapted to upset the balance of the bridge in response thereto, means for impressing received modulated waves on said element, a circuit arrangement selective to a band of frequencies including the frequency of the local source connected to the fixed conjugate points to derive therefrom modulated intermediate frequency oscillations having the carrier frequency of the local source and modulations corresponding to those of the received wave, at least one arm of the bridge containing a manually variable impedance element adapted to upset, in a desired amount, the normal balance of the bridge.

9. In combination, a local source of oscillations of a fixed intermediate frequency, a bridge circuit coupled thereto and normally so balanced that between two fixed points in the circuit, conjugate to the points of coupling to said source, the normal potential difference at the local source frequency is zero, the bridge having a plurality of arms at least one of which contains an element responsive to received modulated waves and adapted to upset the balance of the bridge in response thereto, means for impressing received modulated waves on said element, a circuit arrangement selective to a band of frequencies including the frequency of the local source connected to the fixed conjugate points to derive therefrom modulated intermediate frequency oscillations having the carrier frequency of the local source and modulations corresponding to those of the received Wave, the relative amplitudes of modulated and unmodulated components of said modulated intermediate frequency oscillations depending on the values and arrangement of the bridge elements, an amplifier and a detector associated with said selective circuitarrangement and adapted to derive usable signals from said modulated intermediate frequency oscillations, an automatic volume control arrangement for maintaining the level of said signals at a constant value independent of variations in the strength of said received modulated wave, and manually operable means for altering at will the relative magnitudes of said modulated and unmodulated components to enable the signal level to be altered as desired.

10. The method of transferring modulations of an incoming received carrier wave to a second carrier wave by a single frequency conversion which comprises subjecting the modulated incoming carrier wave and oscillations of the second carrier wave frequency to an interaction characterized by a cube law characteristic and selecting directly from the resultant a band of oscillations in the region of the second carrier Wave frequency of suflicient frequency extent to including the desired modulated sideband components of the second carrier wave but too restrictedl in extentto include osoillations .of the incoming carrier wave frequency.

11. The method of carrier wave transmission comprising subjecting an amplitude modulated carrier wave and an unmodulated carrier wave of different carrier frequency to an interaction having a cube law characteristic whereby there is produced a complex resultant including as one component a Wave having the carrier frequency of the unmodulated carrier wave and having an amplitude modulation corresponding to the square of that of the modulated carrier wave, selecting the said component and subjecting it to a rooting operation to cause the amplitude modulation to more closely simulate that of the original modulated carrier wave.

12. In combination, a receiving circuit, a source "oflocal -oscillations,.'a device having a cubellaw characteristic connected to the receiving circuit "and to the'source of local oscillations whereby received oscillations and local oscillations are subjected to an interaction having a cube law characteristic, means connected to the cube law 'device to select from the resultant of the interaction oscillations of the frequency of the local source carrying a modulation which is a function of that of the received waves, and a router circuit connected to the selecting means for deriving from the selected modulated oscillations, oscillations of the same local source frequency and having an amplitude modulation corresponding to the square root of the amplitude modulations of the selected carrier wave.

RUSSELL S. OHL. 

